Ya , prepsol should be used first , to help lift any contaminants before you start. I may use it again is if you have a car that has been severly undercoated or oiled and you can see the oils bleeding into the primer, or you are sure that there has been a contaminant on the surface of the primer ( oil spray, oily hands, armourall or wax contact). Sometimes this will happen if the car sits in primer for an extended period of time.